1. Field of the Invention
The present application pertains to applications where a linear transconductor is required (i.e., a linear voltage to current conversion), and more specifically, a radio frequency (RF) mixer with improved intermodulation product suppression.
2. Background
Mixers are used for up-conversion of baseband (or low frequency) signals and for down-conversion of RF signals. Intermodulation suppression is useful for a RF receiver wherein the mixer serves to downconvert the RF signal to baseband. The mixer stage performs a voltage to current conversion. This conversion is nonlinear and, as a result, may produce intermodulation distortion.
Intermodulation distortion is caused by two spurious signals which have a small frequency offset between them, mixing together (due to the nonlinearity of the device characteristics). If tone 1 is referred to as f1 and tone 2 is referred to as f2, the intermodulation product 2f2-f1 or 2f1-f2 may lie close to the RF wanted signal and degrade the performance of the receiver. This tone is called the third order intermodulation product.
A receiver or a component of a receiver may be characterized by a third order distortion figure of merit referred to as “third order input intercept point” (IIP3). A third order input intercept point may he defined as the point at which the power in the third-order intermodulation (or distortion) product and the fundamental frequency (or tone) intersect. (See FIG. 1A). The power in the intermodulation product is proportional to the cubic power of the input signal amplitude. For an ideal amplifier (with no distortion) the IIP3 point may be at infinity. The higher the IIP3 point, the better the linearity or the distortion performance for the receiver.
Linearity and noise factor (NF) of a mixer may be dominated by the transconductor stage which translates a voltage input to a current. The mixer transconductor may be linearized with source degeneration (adding a resistor at the source of the transconductor devices), but this will occur at the expense of increased noise and reduction of gain of the mixer (degradation of noise factor).
The present invention is directed to overcoming limitations of the prior art and providing a transconductor with improved intermodulation suppression. Linearization is achieved with a feed-forward technique which minimizes compromise on the gain and noise figure.